KVM: x86 emulator: handle undecoded rex.b with r/m = 5 in certain cases

x86_64 does not decode rex.b in certain cases, where the r/m field = 5.
Signed-off-by: Avi Kivity <avi@qumranet.com>

arch/x86/kvm/x86_emulate.c

@@ -750,6 +750,7 @@ static int decode_modrm(struct x86_emulate_ctxt *ctxt,
 
 			switch (base_reg) {
 			case 5:
+			case 13:
 				if (c->modrm_mod != 0)
 					c->modrm_ea += c->regs[base_reg];
 				else
@@ -767,6 +768,7 @@ static int decode_modrm(struct x86_emulate_ctxt *ctxt,
 			}
 			break;
 		case 5:
+		case 13:
 			if (c->modrm_mod != 0)
 				c->modrm_ea += c->regs[c->modrm_rm];
 			else if (ctxt->mode == X86EMUL_MODE_PROT64)